U.S. patent application number 10/535053 was filed with the patent office on 2007-02-15 for mass spectrometer.
This patent application is currently assigned to MICROMASS UK LIMITED. Invention is credited to Robert Harold Bateman, Gareth Rhys Jones.
Application Number | 20070034796 10/535053 |
Document ID | / |
Family ID | 32328062 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070034796 |
Kind Code |
A1 |
Jones; Gareth Rhys ; et
al. |
February 15, 2007 |
Mass spectrometer
Abstract
A magnetic sector mass spectrometer is disclosed comprising an
ion detector (11) wherein a reflecting electrode (13) is used to
divide an ion beam in the direction of mass dispersion into two
separate ion beams. The two ion beams are directed onto two
detectors which preferably comprise two or more conversion dynodes
(15a, 15b) and two or more corresponding microchannel plate
detectors (14a, 14b) to detect electrons produced by the conversion
dynodes (15a, 15b). If the signal from the two detectors differs
substantially then the ion beam can be determined to include
interference ions. Conversely, if the signal from the two detectors
is substantially the same then the ion beam can be determined to be
substantially free from interference ions.
Inventors: |
Jones; Gareth Rhys;
(Cheshire, GB) ; Bateman; Robert Harold;
(Cheshire, GB) |
Correspondence
Address: |
WATERS INVESTMENTS LIMITED;C/O WATERS CORPORATION
34 MAPLE STREET - LG
MILFORD
MA
01757
US
|
Assignee: |
MICROMASS UK LIMITED
Atlas Park Simonsway
Manchester
GB
M22 5PP
|
Family ID: |
32328062 |
Appl. No.: |
10/535053 |
Filed: |
October 28, 2003 |
PCT Filed: |
October 28, 2003 |
PCT NO: |
PCT/GB03/04658 |
371 Date: |
July 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60427558 |
Nov 20, 2002 |
|
|
|
Current U.S.
Class: |
250/300 ;
250/281 |
Current CPC
Class: |
H01J 49/30 20130101 |
Class at
Publication: |
250/300 ;
250/281 |
International
Class: |
H01J 49/30 20070101
H01J049/30; H01J 49/32 20070101 H01J049/32 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2002 |
GB |
0226715.1 |
Claims
1. A magnetic sector mass spectrometer comprising: a magnetic
sector mass analyser; a collector slit arranged downstream of said
magnetic sector mass analyser; a device arranged downstream of said
collector slit for dividing an ion beam transmitted through said
collector slit into at least a first ion beam and a second ion
beam; a first detector for measuring the intensity of at least a
portion of said first ion beam; and a second detector for measuring
the intensity of at least a portion of said second ion beam.
2. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said ion beam has a first direction and a second orthogonal
direction.
3. A magnetic sector mass spectrometer as claimed in claim 2,
wherein ions in said ion beam are dispersed according to their mass
to charge ratio in said first direction so that the mass to charge
ratio of ions in said ion beam varies along said first
direction.
4. A magnetic sector mass spectrometer as claimed in claim 2,
wherein ions in said ion beam are substantially not dispersed
according to their mass to charge ratio in said second direction so
that the mass to charge ratio of ions in said ion beam is
substantially constant along said second direction.
5. A magnetic sector mass spectrometer as claimed in claim 1,
wherein, in use, said first and second detectors measure the
intensities of at least a portion of said first and second ion
beams at substantially the same time.
6. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said magnetic sector mass spectrometer comprises a single
focusing magnetic sector mass spectrometer.
7. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said magnetic sector mass spectrometer comprises a double
focussing magnetic sector mass spectrometer.
8. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said device comprises an electrode which causes ions to be
reflected or deflected onto said first and second detectors.
9. A magnetic sector mass spectrometer as claimed in claim 8,
wherein said electrode comprises a finely edged blade.
10. A magnetic sector mass spectrometer as claimed in claim 8,
wherein said electrode comprises a wedge shaped electrode.
11. A magnetic sector mass spectrometer as claimed in claim 8,
wherein said electrode comprises an edge and wherein, in use,
analyte ions in said ion beam approaching said edge are arranged so
that they are disposed substantially uniformly and/or symmetrically
relative to said edge.
12. A magnetic sector mass spectrometer as claimed in claim 8,
wherein said electrode comprises an edge and wherein, in use,
interference ions in said ion beam approaching said edge are
arranged so that they are disposed substantially non-uniformly
and/or asymmetrically relative to said edge.
13. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising an Electron Impact ("EI") ion source.
14. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a Chemical Ionisation ("CI") ion source.
15. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising an ion source selected from the group consisting
of: (i) an Electrospray ("ESI") ion source; (ii) an Atmospheric
Pressure Chemical Ionisation ("APCI") ion source; (iii) an
Atmospheric Pressure Photo Ionisation ("APPI") ion source; (iv) a
Matrix Assisted Laser Desorption Ionisation ("MALDI") ion source;
(v) a Laser Desorption Ionisation ("LDI") ion source; (vi) an
Inductively Coupled Plasma ("ICP") ion source; (vii) a Fast Atom
Bombardment ("FAB") ion source; (viii) a Liquid Secondary Ions Mass
Spectrometry ("LSIMS") ion source; (ix) a Field Ionisation ("FI")
ion source; and (x) a Field Desorption ("FD") ion source.
16. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a continuous ion source.
17. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a pulsed ion source.
18. A magnetic sector mass spectrometer as claimed in claim 13,
wherein, in use, a voltage difference is maintained between said
device and said ion source selected from the group consisting of:
(i) 0-100 V; (ii) 100-200 V; (iii) 200-300 V; (iv) 300-400 V; (v)
400-500 V; (vi) 500-600 V; (vii) 600-700 V; (viii) 700-800 V; (ix)
800-900 V; (x) 900-1000 V; and (xi) >1000 V.
19. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a processor, said processor determining, in use,
the intensity of at least a portion of said first ion beam relative
to the intensity of at least a portion of said second ion beam.
20. A magnetic sector mass spectrometer as claimed in claim 1,
wherein if the intensity of at least a portion of said first ion
beam differs from the intensity of at least a portion of said
second ion beam by .gtoreq.x %, then a determination is made that
said ion beam includes a significant proportion of interference
ions, wherein x is selected from the group consisting of: (i) 1;
(ii) 2; (iii) 3; (iv) 4; (v) 5; (vi) 6; (vii) 7; (viii) 8; (ix) 9;
(x) 10; (xi) 15; (xii) 20; (xiii) 25; (xiv) 30; (xv) 35; (xvi) 40;
(xvii) 45; (xviii) 50; (xix) 55; (xx) 60; (xxi) 65; (xxii) 70;
(xxiii) 75; (xxiv) 80; (xxv) 85; (xxvi) 90; (xxvii) 95; (xxviii)
100; and (xxix) >100.
21. A magnetic sector mass spectrometer as claimed in claim 1,
wherein if the intensity of at least a portion of said second ion
beam differs from the intensity of at least a portion of said first
ion beam by .gtoreq.x %, then a determination is made that said ion
beam includes a significant proportion of interference ions,
wherein x is selected from the group consisting of: (i) 1; (ii) 2;
(iii) 3; (iv) 4; (v) 5; (vi) 6; (vii) 7; (viii) 8; (ix) 9; (x) 10;
(xi) 15; (xii) 20; (xiii) 25; (xiv) 30; (xv) 35; (xvi) 40; (xvii)
45; (xviii) 50; (xix) 55; (xx) 60; (xxi) 65; (xxii) 70; (xxiii) 75;
(xxiv) 80; (xxv) 85; (xxvi) 90; (xxvii) 95; (xxviii) 100; and
(xxix) >100.
22. A magnetic sector mass spectrometer as claimed in claim 1,
wherein if within a time t the number of ions detected by said
first detector differs from the number of ions detected by said
second detector by .gtoreq.y standard deviations of the total
number of ions detected by said first and second detectors during
said time t, then a determination is made that said ion beam
includes a significant proportion of interference ions, wherein y
is selected from the group consisting of: (i) 0.25; (ii) 0.5; (iii)
0.75; (iv) 1.0; (v) 1.25; (vi) 1.5; (vii) 1.75; (viii) 2.0; (ix)
2.25; (x) 2.5; (xi) 2.75; (xii) 3.0; (xiii) 3.25; (xiv) 3.5; (xv)
3.75; (xvi) 4.0; and (xvii) >4.0.
23. A magnetic sector mass spectrometer as claimed in claim 1,
wherein signals from said first and second detectors are summed to
produce a combined signal and wherein said combined signal is
multiplied by a weighting factor.
24. A magnetic sector mass spectrometer as claimed in claim 23,
wherein said weighting factor: (i) does not substantially attenuate
said combined signal when the signal from said first detector
substantially equals the signal from said second detector; and/or
(ii) substantially attenuates said combined signal when the signal
from said first detector substantially differs from the signal from
said second detector.
25. A magnetic sector mass spectrometer as claimed in claim 23,
wherein said weighting factor is of the form exp(-ky.sup.n) wherein
k and n are constants and wherein within a time t the number of
ions detected by said first detector differs from the number of
ions detected by said second detector by y standard deviations of
the total number of ions detected by said first and second
detectors during said time t.
26. A magnetic sector mass spectrometer as claimed in claim 25,
wherein k is selected from the group consisting of: (i) 0.5-2.0;
(ii) 0.6-1.8; (iii) 0.7-1.6; (iv) 0.8-1.4; (v) 0.9-1.2; (vi)
0.95-1.1; and (vii) 1.
27. A magnetic sector mass spectrometer as claimed in claim 25,
wherein n is selected from the group consisting of: (i) 1.0-3.0;
(ii) 1.2-2.8; (iii) 1.4-2.6; (iv) 1.6-2.4; (v) 1.8-2.2; (vi)
1.9-2.1; and (vii) 2.
28. A magnetic sector mass spectrometer as claimed in claim 1,
wherein if a determination is made that said ion beam includes a
significant proportion of interference ions then signals from said
first and/or said second detectors are discarded or are otherwise
deemed to be relatively inaccurate.
29. A magnetic sector mass spectrometer as claimed in claim 1,
wherein if a determination is made that said ion beam does not
include a significant proportion of interference ions then signals
from said first and second detectors are summed or are otherwise
deemed to be relatively accurate.
30. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a lens arranged downstream of said collector
slit.
31. A magnetic sector mass spectrometer as claimed in claim 30,
wherein said lens refocuses the image of said collector slit onto
said device.
32. A magnetic sector mass spectrometer as claimed in claim 30,
wherein said lens substantially collimates said ion beam.
33. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising a screening tube for guiding ions onto said
device.
34. A magnetic sector mass spectrometer as claimed in claim 33,
wherein said screening tube is arranged between said collector slit
and said device.
35. A magnetic sector mass spectrometer as claimed in claim 33,
wherein said screening tube shields said ion beam from voltages
applied to said first and/or said second detector.
36. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said first detector comprises one, two, three, four, five,
six, seven, eight, nine, ten or more than ten microchannel plate
detectors.
37. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said first detector comprises one, two, three, four, five,
six, seven, eight, nine, ten or more than ten conversion dynode(s)
for generating electrons in response to ions impinging upon said
conversion dynode(s).
38. A magnetic sector mass spectrometer as claimed in claim 37,
further comprising one or more electron multipliers and/or one or
more microchannel plate detectors for detecting electrons generated
by said conversion dynode(s).
39. A magnetic sector mass spectrometer as claimed in claim 37,
further comprising one or more scintillators and/or one or more
phosphers upon which electrons generated by said conversion
dynode(s) are received in use and wherein said one or more
scintilators and/or said one or more phosphers generate photons in
response to receiving electrons.
40. A magnetic sector mass spectrometer as claimed in claim 39,
further comprising one or more photo-multiplier tubes and/or one or
more photo-sensitive solid state detectors for detecting said
photons.
41. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said second detector comprises one, two, three, four, five,
six, seven, eight, nine, ten or more than ten microchannel plate
detectors.
42. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said second detector comprises one, two, three, four, five,
six, seven, eight, nine, ten or more than ten conversion dynode(s)
for generating electrons in response to ions impinging upon said
conversion dynode(s).
43. A magnetic sector mass spectrometer as claimed in claim 42,
further comprising one or more electron multipliers and/or one or
more microchannel plate detectors for detecting electrons generated
by said conversion dynode(s).
44. A magnetic sector mass spectrometer as claimed in claim 42,
further comprising one or more scintillators and/or one or more
phosphers upon which electrons generated by said conversion
dynode(s) are received in use and wherein said one or more
scintilators and/or said one or more phosphers generate photons in
response to receiving electrons.
45. A magnetic sector mass spectrometer as claimed in claim 44,
further comprising one or more photo-multiplier tubes and/or one or
more photo-sensitive solid state detectors for detecting said
photons.
46. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising an additional detector arranged upstream of said
first and second detectors.
47. A magnetic sector mass spectrometer as claimed in claim 46,
wherein said additional detector comprises a conversion dynode.
48. A magnetic sector mass spectrometer as claimed in claim 47,
wherein in a mode of operation at least a portion of an ion beam is
deflected onto said conversion dynode and wherein said conversion
dynode generates electrons in response thereto.
49. A magnetic sector mass spectrometer as claimed in claim 48,
further comprising one or more electron multipliers and/or one or
more microchannel plate detectors for receiving electrons generated
by said conversion dynode.
50. A magnetic sector mass spectrometer as claimed in claim 48,
further comprising one or more scintillators and/or one or more
phosphers upon which electrons generated by said conversion dynode
are received in use and wherein said one or more scintillators
and/or said one or more phosphers generate photons in response to
receiving electrons.
51. A magnetic sector mass spectrometer as claimed in claim 50,
further comprising one or more photo-multiplier tubes and/or one or
more photo-sensitive solid state detectors for detecting said
photons.
52. A magnetic sector mass spectrometer as claimed in claim 1,
wherein the gain of said first and/or said second detector can be
independently adjusted.
53. A magnetic sector mass spectrometer as claimed in claim 52,
wherein said first and second detectors are powered by
independently adjustable power supplies.
54. A magnetic sector mass spectrometer as claimed in claim 1,
wherein said first and second detectors further comprise one or
more Analogue to Digital Converters and/or one or more ion counting
detectors.
55. A magnetic sector mass spectrometer as claimed in claim 1,
further comprising adjustment means for centering said ion beam on
to said device.
56. A magnetic sector mass spectrometer as claimed in claim 55,
wherein said adjustment means comprises at least one deflecting
electrode downstream of said collector slit, said deflecting
electrode being arranged to move said ion beam relative to said
device.
57. A method of mass spectrometry comprising: transmitting an ion
beam through a magnetic sector mass analyser and a collector slit
arranged downstream of said magnetic sector mass analyser; dividing
said ion beam downstream of said collector slit into at least a
first ion beam and a second ion beam; measuring the intensity of at
least a portion of said first ion beam with a first detector; and
measuring the intensity of at least a portion of said second ion
beam with a second detector.
58. A method of mass spectrometry as claimed in claim 57, wherein
said ion beam has a first direction and a second orthogonal
direction.
59. A method of mass spectrometry as claimed in claim 58, wherein
ions in said ion beam are dispersed according to their mass to
charge ratio in said first direction so that the mass to charge
ratio of ions in said ion beam varies along said first
direction.
60. A method of mass spectrometry as claimed in claim 58, wherein
ions in said ion beam are substantially not dispersed according to
their mass to charge ratio in said second direction so that the
mass to charge ratio of ions in said ion beam is substantially
constant along said second direction.
61. A method of mass spectrometry as claimed in claim 57, wherein,
in use, said first and second detectors measure the intensities of
at least a portion of said first and second ion beams at
substantially the same time.
62. A method of mass spectrometry as claimed in claim 57, further
comprising determining the intensity of at least a portion of said
first ion beam relative to the intensity of at least a portion of
said second ion beam.
63. A method of mass spectrometry as claimed in claim 57, wherein
if the intensity of at least a portion of said first ion beam
differs from the intensity of at least a portion of said second ion
beam by .gtoreq.x %, then a determination is made that said ion
beam includes a significant proportion of interference ions,
wherein x is selected from the group consisting of: (i) 1; (ii) 2;
(iii) 3; (iv) 4; (v) 5; (vi) 6; (vii) 7; (viii) 8; (ix) 9; (x) 10;
(xi) 15; (xii) 20; (xiii) 25; (xiv) 30; (xv) 35; (xvi) 40; (xvii)
45; (xviii) 50; (xix) 55; (xx) 60; (xxi) 65; (xxii) 70; (xxiii) 75;
(xxiv) 80; (xxv) 85; (xxvi) 90; (xxvii) 95; (xxviii) 100; and
(xxix) >100.
64. A method of mass spectrometry as claimed in claim 57, wherein
if the intensity of at least a portion of said second ion beam
differs from the intensity of at least a portion of said first ion
beam by .gtoreq.x %, then a determination is made that said ion
beam includes a significant proportion of interference ions,
wherein x is selected from the group consisting of: (i) 1; (ii) 2;
(iii) 3; (iv) 4; (v) 5; (vi) 6; (vii) 7; (viii) 8; (ix) 9; (x) 10;
(xi) 15; (xii) 20; (xiii) 25; (xiv) 30; (xv) 35; (xvi) 40; (xvii)
45; (xviii) 50; (xix) 55; (xx) 60; (xxi) 65; (xxii) 70; (xxiii) 75;
(xxiv) 80; (xxv) 85; (xxvi) 90; (xxvii) 95; (xxviii) 100; and
(xxix) >100.
65. A method of mass spectrometry as claimed in claim 57, wherein
if within a time t the number of ions detected by said first
detector differs from the number of ions detected by said second
detector by .gtoreq.y standard deviations of the total number of
ions detected by said first and second detectors during said time
t, then a determination is made that said ion beam includes a
significant proportion of interference ions, wherein y is selected
from the group consisting of: (i) 0.25; (ii) 0.5; (iii) 0.75; (iv)
1.0; (v) 1.25; (vi) 1.5; (vii) 1.75; (viii) 2.0; (ix) 2.25; (x)
2.5; (xi) 2.75; (xii) 3.0; (xiii) 3.25; (xiv) 3.5; (xv) 3.75; (xvi)
4.0; and (xvii) >4.0.
66. A method of mass spectrometry as claimed in claim 57, further
comprising: summing signals from said first and second detectors to
produce a combined signal; and multiplying said combined signal by
a weighting factor.
67. A method of mass spectrometry as claimed in claim 66, wherein
said weighting factor: (i) does not substantially attenuate said
combined signal when the signal from said first detector
substantially equals the signal from said second detector; and/or
(ii) substantially attenuates said combined signal when the signal
from said first detector substantially differs from the signal from
said second detector.
68. A method of mass spectrometry as claimed in claim 66, wherein
said weighting factor is of the form exp(-ky.sup.n) wherein k and n
are constants and wherein within a time t the number of ions
detected by said first detector differs from the number of ions
detected by said second detector by y standard deviations of the
total number of ions detected by said first and second detectors
during said time t.
69. A method of mass spectrometry as claimed in claim 68, wherein k
is selected from the group consisting of: (i) 0.5-2.0; (ii)
0.6-1.8; (iii) 0.7-1.6; (iv) 0.8-1.4; (v) 0.9-1.2; (vi) 0.95-1.1;
and (vii) 1.
70. A method of mass spectrometry as claimed in claim 68, wherein n
is selected from the group consisting of: (i) 1.0-3.0; (ii)
1.2-2.8; (iii) 1.4-2.6; (iv) 1.6-2.4; (v) 1.8-2.2; (vi) 1.9-2.1;
and (vii) 2.
71. A method of mass spectrometry as claimed in claim 57, wherein
if a determination is made that said ion beam includes a
significant proportion of interference ions then signals from said
first and/or said second detectors are discarded or are otherwise
deemed to be relatively inaccurate.
72. A method of mass spectrometry as claimed in claim 57, wherein
if a determination is made that said ion beam does not include a
significant proportion of interference ions then signals from said
first and second detectors are summed or are otherwise deemed to be
relatively accurate.
Description
[0001] The present invention relates to a magnetic sector mass
spectrometer and a method of a mass spectrometry.
[0002] Magnetic sector mass spectrometers are commonly used for
target compound trace analysis, accurate mass measurements, isotope
ratio measurements and fundamental ion chemistry studies. Magnetic
sector mass spectrometers are arranged to transmit ions having a
particular mass to charge ratio to an ion detector. As described in
more detail below, ions pass through the magnetic sector mass
analyser on a substantially circular trajectory. A magnetic sector
mass analyser may more accurately be described as being an ion
momentum analyser but if the initial energies of the ions are
substantially the same then the ions will become separated
according to their mass to charge ratio.
[0003] Ions having a mass m and a charge ze when accelerated
through an electrical potential difference V will attain a velocity
v and possess a kinetic energy .epsilon. wherein: = zeV = mv 2 2
##EQU1## and hence: v 2 = 2 .times. zeV m ##EQU2##
[0004] Ions with a charge ze moving through a magnetic field B with
a velocity v will be subject to a Lorentz force F in a direction
orthogonal to both the direction of the magnetic field and the
direction of travel of the ions. The Lorentz force F will exert a
centripetal force on the ions causing them to travel in a circular
trajectory having a radius r.sub.m. The Lorentz force F is: F =
Bzev = mv 2 r m ##EQU3##
[0005] Accordingly, the mass to charge ratio of the ions travelling
through the magnetic field is given by: m ze = Bvr m v 2 ##EQU4##
and hence: ( mv ze ) = Br m ##EQU5## Therefore, eliminating v.sup.2
from the above equation for mass to charge ratio gives: m .times.
ze = Bvr .times. m .times. ( m .times. 2 .times. .times. zeV ) =
.times. Br .times. m .times. 2 .times. .times. V .times. ( mv ze )
##EQU6## m ze = B 2 .times. r m 2 2 .times. V ##EQU6.2##
[0006] From this it can be seen that the values of the magnetic
field B and the potential difference V may be set so that ions
having a particular mass to charge ratio received from an ion
source are transmitted by the magnetic sector to the ion detector.
In this manner the magnetic sector acts as a mass to charge ratio
filter. Accordingly, a mass spectrum can be recorded by scanning
either the magnetic field B and/or the potential difference V.
[0007] For some applications multiple ion detectors may be provided
so that ions having different mass to charge ratios may be
simultaneously recorded wherein each ion takes a different
trajectory through the magnetic sector. Alternatively, an array of
detectors may be used to simultaneously record a portion of the
mass spectrum.
[0008] According to another arrangement, the magnetic field may be
maintained substantially constant so that ions are dispersed
according to their momentum. The momentum .rho. of an ion having a
mass m, velocity v and kinetic energy .epsilon. is given by:
.rho.=mv= {square root over (2m.epsilon.)}
[0009] Therefore, ions with a constant kinetic energy .epsilon.
are, in effect, dispersed according to their mass.
[0010] The shape of a magnetic sector can be designed to have ion
directional focusing properties. A magnetic sector mass analyser
may be designed to have a particular combination of mass dispersion
and directional focusing characteristics in the direction of mass
dispersion.
[0011] A conventional single focusing magnetic sector mass
spectrometer comprises an ion source, a magnetic sector mass
analyser and a collector slit. The ion source has a finite emitting
region or slit width which defines the width of the ion beam
emitted from the ion source. The magnetic sector mass analyser may
have convergent directional focusing characteristics in order to
focus the ions to an image point in a focal plane downstream of the
magnetic sector mass analyser. In a single focusing magnetic sector
mass spectrometer an ion collector slit is positioned at the image
point of the ion source slit. The directional focusing
characteristics of the magnetic sector mass analyser can be
designed to a very high order. However, the imaging properties of
the magnetic sector mass analyser will be limited by any spread in
the initial energy of the ions.
[0012] The mass dispersion coefficient D.sub.m of a single focusing
magnetic sector mass spectrometer is proportional to the radius of
curvature r.sub.m of the ion beam trajectory in the magnetic field.
The spatial separation y of two ions having different masses of
mean mass m and mass difference .DELTA.m is related to the mass
dispersion coefficient D.sub.m and is: y = D m .times. .DELTA.
.times. .times. m m ##EQU7##
[0013] The ion beam width w.sub.b at the image position downstream
of the magnetic sector mass analyser is related to the ion source
slit width w.sub.s, the image lateral magnification M and the sum
of the imaging aberration coefficients .SIGMA..alpha. as follows:
w.sub.b=Mw.sub.s+.SIGMA..alpha.
[0014] The mass resolving power m/.DELTA.m for a collector slit
having a collector slit width w.sub.c is given by: m .DELTA.
.times. .times. m = D m w b + w c = D m Mw g + w c + .alpha.
##EQU8##
[0015] Thus, the mass dispersion coefficient D.sub.m, the ion
source slit width w.sub.s and the collector slit width w.sub.c are
the most significant parameters in determining the mass resolution
of a magnetic sector mass spectrometer. However, the ultimate mass
resolution will be limited by the sum of the imaging
aberrations.
[0016] As discussed above, magnetic sectors employing a constant
magnetic field disperse ions with respect to the momentum of the
ions and hence with respect to the mass of the ions if the ions are
mono-energetic. However, ions will not normally be mono-energetic
and will often have a range of kinetic energies depending upon the
particular type of ion source used to generate the ions. The spread
in ion energies acts to broaden the ion beam width w.sub.b at the
image position and this typically becomes the limiting factor in
achieving high resolution.
[0017] Momentum dispersion may be considered as comprising a
combination of mass dispersion and energy dispersion. Electric
sectors are known which will disperse ions according to their
energy. Accordingly, if an electric sector is combined with a
magnetic sector then the overall energy dispersion of the ions can
be modified. Double focusing magnetic sector mass analysers are
known comprising a combination of a magnetic sector mass analyser
and one or more electric sectors wherein directional focusing is
provided and wherein the overall energy dispersion is zero. If the
double focussing magnetic sector mass analyser comprises an
electric sector having an energy dispersion D.sub.el, a magnetic
sector having an energy dispersion D.sub.e2 and wherein the image
magnification is M.sub.2 then the overall energy dispersion D.sub.e
of the double focussing magnetic sector mass spectrometer is:
D.sub.e=M.sub.2D.sub.e1+D.sub.e2
[0018] The electric sector may precede or follow the magnetic
sector or alternatively two smaller electric sectors may be
provided, one upstream and the other downstream of the magnetic
sector. As long as the overall energy dispersion D.sub.e is zero
then the arrangement may be considered as being a double focusing
magnetic sector mass analyser. A combination of magnetic and
electric sectors can be arranged which do not suffer from the image
broadening problems associated with a single focussing magnetic
sector mass spectrometers. Accordingly, double focussing magnetic
sector mass spectrometers are capable of achieving much higher
resolutions than single focussing magnetic sector mass
spectrometers.
[0019] The combination of a magnetic sector and one or more
electric sectors to provide a double focusing magnetic sector mass
spectrometer allows sufficient degrees of freedom in the choice of
design to allow relatively high order focusing to be achieved.
Double focusing magnetic sector mass spectrometers in which all
second order directional and energy focussing terms are
approximately or substantially zero are known and such mass
spectrometers can achieve resolving powers in excess of 150,000
according to the 10% valley definition (which is described in more
detail below).
[0020] The mass resolution for a peak width in mass units of
.DELTA.m, at mass m, is m/.DELTA.m. If the peak width W.sub.pk is
measured at its base then theoretically the mass resolution
m/.DELTA.m for an ion beam width w.sub.b and a collector slit width
w.sub.c is given by: m .DELTA. .times. .times. m = D m w pk = D m (
w b + w c ) ##EQU9##
[0021] However, it is not practical to measure the peak width at
its base and so conventionally the peak width is measured at 5% of
the peak height. The peak width as measured at 5% of its height is
used to calculate the resolution. This is known as the 10% valley
definition of resolution since if two peaks of different mass, but
equal intensity or height, were to overlap or intersect at a point
equal to 5% of their height then the resultant peak profile would
exhibit two peaks with a valley between them which is 10% of the
height of either of the peaks. For example, if a magnetic sector
mass spectrometer were to have a mass resolution of 1000 according
to the 10% valley definition then two equal intensity peaks with
masses 1000 and 1001 would be resolved such that the valley between
the peaks of the resultant peak profile would be 10% of the height
of either of the peaks.
[0022] As discussed above the spatial separation y of two ions
having different masses of mean mass m and mass difference .DELTA.m
is related to the dispersion coefficient D.sub.m. This relationship
can be used to express the real width of an ion beam w.sub.b at the
collector slit in terms of the fractional mass difference of the
ions .DELTA.m/m as follows: .DELTA. .times. .times. m m = w b D m
##EQU10##
[0023] The term for the fractional mass difference of the ions
.DELTA.m/m is dimensionless and it is typically expressed in parts
per million (ppm) where: .DELTA. .times. .times. m m = w b D m
.times. 10 6 .times. .times. ppm ##EQU11##
[0024] Accordingly, the beam width w.sub.b may be expressed in ppm
of mass when the dispersion coefficient D.sub.m of the mass
spectrometer is known. The collector slit width w.sub.c may also be
expressed in ppm of mass as follows: .DELTA. .times. .times. m m =
w c D m .times. 10 6 .times. .times. ppm ##EQU12##
[0025] When an ion beam of width w.sub.b is swept across a
collector slit of width w.sub.c and the transmitted ions are
detected and recorded, then the recorded peak profile will have a
width W.sub.pk where: w.sub.pk=w.sub.b+w.sub.c
[0026] The peak width w.sub.pk may also be expressed in terms of
ppm of mass: .DELTA. .times. .times. m m = w pk D m .times. 10 6
.times. .times. ppm ##EQU13##
[0027] The inverse of mass resolution m/.DELTA.m of the mass
analyser gives the mass resolving power .DELTA.m/m. Therefore, the
mass resolving power can be considered as the peak width expressed
in ppm of mass.
[0028] The capacity of double focusing magnetic sector mass
spectrometers for high resolution results in their use for accurate
mass measurements and for highly specific target compound trace
analysis by a technique known as High Resolution Selective Ion
Recording ("HR-SIR"). Conventional High Resolution Selective Ion
Recording techniques use a double focusing magnetic sector mass
spectrometer to select and record the response from target
compounds at high resolution and with a high sensitivity. The high
resolution enables background chemical masses to be effectively
eliminated and consequently allows a lower detection level to be
achieved. High Resolution Selective Ion Recording therefore
provides a higher duty cycle and hence improved sensitivity
compared with other conventional techniques.
[0029] The detection and quantification of polychlorinated
dibenzo-p-dioxins, and in particular 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin ("2,3,7,8-TCDD") is a particularly important
application of double focusing magnetic sector mass spectrometers.
Despite extensive clean-up procedures, samples may still contain
compounds such as polychlorinated biphenyls and benzylphenylethers
which will have the same nominal masses as the compounds of
interest. Samples are conventionally spiked with a known amount of
a .sup.13C isotope labelled form of 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin introduced via gas chromatography and recorded by
high resolution mass spectrometry. The measurement is quantified by
comparison of the native dioxin response to that of the .sup.13C
labelled form and verified by confirmation of the ratio of the
major isotopes of both the native and the .sup.13C labelled
dioxins. At a resolving power of 10,000 (10% valley definition) the
conventional detection level for 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin is approximately 1 femto-gram, or 3 atto-mole, in
the absence of other interfering components.
[0030] A magnetic sector mass spectrometer with a single ion
detector may be used to record a mass spectrum by scanning and
sequentially detecting different mass peaks. The duty cycle for
recording each mass in the mass spectrum is generally relatively
poor and the higher the resolution or the wider the mass range the
poorer the duty cycle becomes. Unlike quadrupole mass filters, a
magnetic sector mass analyser may be designed to record the signal
from ions having several different masses simultaneously. This is
commonly referred to as parallel detection.
[0031] Multiple detectors provide a means of accurately recording
the relative abundance of two or more different masses
simultaneously. The simultaneous accurate recording of the relative
abundances of, for example, two isotopes is particularly accurate
since this technique is substantially unaffected by fluctuations or
drift in the ionisation source or in rapidly changing sample
concentrations which is often encountered, for example, in
chromatography. Magnetic sector mass spectrometers incorporating
multiple collector slits and corresponding separate discrete ion
detectors may therefore be used to make accurate isotope ratio
determinations. Different ion detectors are required to record
different masses but only at a low resolution of, for example,
200-300 (10% valley definition)
[0032] According to another conventional arrangement an array
detector allows simultaneous acquisition over a range of masses
thereby improving the duty cycle when used to record a mass
spectrum. Array detectors employing high-density arrays of discrete
charge sensitive detectors or single ion position sensitive
detectors are very sensitive but are usually limited in size. Such
array detectors are positioned along the focal plane of the mass
spectrometer and therefore replace the collector slit which is
otherwise normally used in conjunction with an ion detector in a
magnetic sector mass spectrometer. Each separate detector in the
array therefore substitutes for the collector slit and these
separate detectors determine the resolution of the mass
spectrometer. Since the detector is required to record several
masses at the same time in practice it can only be operated at up
to a medium resolution e.g. up to a resolution of about 2000 (10%
valley definition). Such a resolution is still far too low for the
analysis of polychlorinated dibenzo-p-dioxins.
[0033] Conventional High Resolution Selected Ion Recording
techniques for the detection of traces of 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin involve repetitive rapid switching to at least
four different masses at high resolution and recording the signal
response for all four masses. This is commonly carried out at a
mass resolution of around 10,000 (10% valley definition) to ensure
that other isobaric components eluting from the gas chromatography
column are separated out. In practice, an additional reference
material is usually continuously infused into the ion source of the
mass spectrometer so that an additional reference mass peak, which
is close in mass to that of the trace compound to be analysed, is
continuously present. The additional reference mass is included in
the switching sequence so that any drift in the mass scale can be
monitored and corrected for. The drift in the mass scale can be
monitored by scanning across the reference peak to determine any
shift in the peak centre. If drift in the mass scale is not
monitored for then the switching to the peak top of each of the
four masses of interest could not be performed with the necessary
degree of certainty. It is also known to switch to the reference
peak at a second time in each sequence to verify that the switching
operation is working correctly and accurately. This procedure
ensures accurate switching at a resolution of 10,000 (10% valley
definition). However, although this procedure is sensitive it does
not ensure that all of the ions detected are actually solely ions
of the target compound of interest. Accordingly, interference ions
may also be inadvertently detected.
[0034] Interference ions may be detected due to, for example,
contamination materials in the ion source, reference material,
bleed material from the gas chromatograph column, or other
co-eluting components from the gas chromatograph which have very
similar mass to charge ratios to the intended analyte ions. These
interference ions may be detected because they may not be fully
separated from the analyte ions even at a resolution of 10,000 (10%
valley definition). Interference ions may also result from
scattering due to ions from other components which are present at
higher abundance colliding with residual gas molecules.
[0035] The main indication of the presence of a major interference
is a distortion of the isotope ratio. Such a distortion is normally
checked for as part of a standard verification procedure. However,
even when interference ions are known to be present by recognising
that the determined isotopic ratio is distorted, the presence of
the interference ions will continue to contribute a background
signal which may obscure the detection of the trace analyte ions of
interest. Switching from peak top to peak top does not provide a
way in itself of verifying whether the detected ions are actually
the ions of interest nor does it help make a determination that the
measured ion signal should be rejected due to the significant
presence of interference ions.
[0036] It is therefore desired to provide an improved magnetic
sector mass spectrometer.
[0037] According to the present invention there is provided a
magnetic sector mass spectrometer comprising a magnetic sector mass
analyser, a collector slit arranged downstream of the magnetic
sector mass analyser and a device arranged downstream of the
collector slit for dividing an ion beam transmitted through the
collector slit into at least a first ion beam and a second ion
beam. The mass spectrometer further comprises a first detector for
measuring the intensity of at least a portion of the first ion beam
and a second detector for measuring the intensity of at least a
portion of the second ion beam.
[0038] The ion beam has a first direction and a second orthogonal
direction. In the preferred embodiment the ions in the ion beam are
dispersed according to their mass to charge ratio in the first
direction so that the mass to charge ratio of ions in the ion beam
varies along the first direction. Preferably, the ions in the ion
beam are substantially not dispersed according to their mass to
charge ratio in the second direction so that the mass to charge
ratio of ions in the ion beam is substantially constant along the
second direction.
[0039] In the preferred embodiment the first and second detectors
measure the intensities of at least a portion of the first and
second ion beams at substantially the same time.
[0040] The mass spectrometer may comprise a single focusing
magnetic sector mass spectrometer or a double focussing magnetic
sector mass spectrometer.
[0041] In the preferred embodiment the device for dividing the ion
beam which is transmitted through the collector slit comprises an
electrode. The electrode is maintained at a potential such that
ions are reflected or deflected onto the first and second
detectors. The electrode preferably comprises a finely edged blade
or a wedge shaped electrode and, in use, analyte ions in the ion
beam approaching the edge may be arranged such that they are
disposed substantially uniformly and/or symmetrically relative to
the edge. Interference ions in the ion beam approaching the edge of
the electrode may be disposed substantially non-uniformly and/or
asymmetrically relative to the edge.
[0042] The magnetic sector mass spectrometer preferably comprises
an Electron Impact ("EI") ion source or a Chemical Ionisation
("CI") ion source. Alternatively, the ion source may be an
Electrospray ("ESI") ion source, an Atmospheric Pressure Chemical
Ionisation ("APCI") ion source, an Atmospheric Pressure Photo
Ionisation ("APPI") ion source, a Matrix Assisted Laser Desorption
Ionisation ("MALDI") ion source, a Laser Desorption Ionisation
("LDI") ion source, an Inductively Coupled Plasma ("ICP") ion
source, a Fast Atom Bombardment ("FAB") ion source, a Liquid
Secondary Ions Mass Spectrometry ("LSIMS") ion source, a Field
Ionisation ("FI") ion source or a Field Desorption ("FD") ion
source. The ion source may be a continuous or pulsed ion
source.
[0043] Preferably, a voltage difference is maintained between the
device for dividing the ion beam and the ion source. The voltage
difference may be 0-100 V, 100-200 V, 200-300 V, 300-400 V, 400-500
V, 500-600 V, 600-700 V, 700-800 V, 800-900 V, 900-1000 V or more
than 1000 V.
[0044] The preferred magnetic sector mass spectrometer may further
comprise a processor for determining the intensity of at least a
portion of the first ion beam relative to the intensity of at least
a portion of the second ion beam. If the intensity of at least a
portion of the first and/or second ion beam differs from the
intensity of at least a portion of the second and/or first ion beam
respectively by greater than or equal to a percent x, then a
determination may be made that the ion beam includes a significant
proportion of interference ions. Preferably, the percent x is
selected from the group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9,
10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90,
95, 100 or more than 100. Alternatively, or in addition, if within
a time t the number of ions detected by the first detector differs
from the number of ions detected by the second detector by greater
than or equal to y standard deviations of the total number of ions
detected by the first and second detectors during the time t, then
a determination may be made that the ion beam includes a
significant proportion of interference ions. Preferably, the number
of standard deviations y is selected from the group consisting of
0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75, 3.0,
3.25, 3.5, 3.75, 4.0 or more than 4.0.
[0045] In a preferred embodiment the signals from the first and
second detectors are summed to produce a combined signal and the
combined signal may be multiplied by a weighting factor.
Preferably, the weighting factor does not substantially attenuate
the combined signal when the signal from the first detector
substantially equals the signal from the second detector.
Additionally, or alternatively, the weighting factor may
substantially attenuate the combined signal when the signal from
the first detector substantially differs from the signal from the
second detector. In one embodiment the weighting factor is of the
form exp (-ky.sup.n), where k and n are constants and within a time
t the number of ions detected by the first detector differs from
the number of ions detected by the second detector by y standard
deviations of the total number of ions detected by the first and
second detectors during the time t. In this embodiment the
difference between the number of ions detected by the first and
second detectors is taken as a positive value, i.e. the modulus of
the difference between the number of ions detected. Preferably, the
constant k is 0.5-2.0, 0.6-1.8, 0.7-1.6, 0.8-1.4, 0.9-1.2, 0.95-1.1
or 1. Preferably, the constant n is 1.0-3.0, 1.2-2.8, 1.4-2.6,
1.6-2.4, 1.8-2.2, 1.9-2.1 or 2.
[0046] In the preferred embodiment, if a determination is made that
the ion beam includes a significant proportion of interference ions
then signals from the first and/or second detectors are discarded
or are otherwise deemed to be relatively inaccurate. Alternatively,
if a determination is made that the ion beam does not include a
significant proportion of interference ions then signals from the
first and second detectors are summed or are otherwise deemed to be
relatively accurate.
[0047] Preferably, the magnetic sector mass spectrometer further
comprises a lens arranged downstream of the collector slit. The
lens may refocus the image of the collector slit onto the device
for splitting the ion beam or may substantially collimate the ion
beam.
[0048] In another embodiment a screening tube is provided for
guiding ions onto the device for splitting the ion beam. The
screening tube is preferably arranged between the collector slit
and the device for splitting the ion beam and may shield the ion
beam from the voltages applied to the first and/or second detector.
Preferably, the first and/or second detector comprises one, two,
three, four, five, six, seven, eight, nine, ten or more than ten
microchannel plate detectors. Additionally, or alternatively, the
first and/or second detector may comprise one, two, three, four,
five, six, seven, eight, nine, ten or more than ten conversion
dynode(s) for generating electrons in response to ions impinging
upon said conversion dynode(s). The mass spectrometer may
additionally comprise one or more electron multipliers and/or one
or more microchannel plate detectors for receiving electrons
generated by the conversion dynode(s). In another embodiment, the
mass spectrometer further comprises one or more scintillators
and/or one or more phosphers upon which the electrons generated by
the conversion dynode(s) are received such that the one or more
scintillators and/or the one or more phosphers generate photons in
response to receiving electrons. The mass spectrometer may also
comprise one or more photo-multiplier tubes and/or one or more
photo-sensitive solid state detectors for detecting the
photons.
[0049] In the preferred embodiment, the magnetic sector mass
spectrometer further comprises an additional detector arranged
upstream of the first and second detectors. This additional
detector may comprise a conversion dynode and in a mode of
operation at least a portion of an ion beam is deflected onto the
conversion dynode of the additional detector such that the
conversion dynode generates electrons in response thereto. The
additional detector may further comprise one or more electron
multipliers and/or one or more microchannel plate detectors for
receiving the electrons generated by the conversion dynode. One or
more scintillators and/or one or more phosphers may also be
provided to receive electrons generated by the conversion dynode
and generate photons in response thereto. These photons may be
detected by one or more photo-multiplier tubes and/or one or more
photo-sensitive solid state detectors.
[0050] Preferably, the gain of the first and/or second detector can
be independently adjusted and in one embodiment the first and
second detectors are powered by independently adjustable power
supplies. The first and second detectors may further comprise one
or more Analogue to Digital Converters and/or one or more ion
counting detectors.
[0051] In another preferred embodiment the magnetic sector mass
spectrometer further comprises adjustment means for centering the
ion beam onto the device for splitting the ion beam. The adjustment
means preferably comprises at least one deflecting electrode
downstream of the collector slit which is arranged to move the ion
beam relative to the device for splitting the ion beam.
[0052] The magnetic sector mass spectrometer according to the
preferred embodiment is particularly suitable for target compound
trace analysis.
[0053] From another aspect the present invention provides a method
of mass spectrometry. The method comprises transmitting an ion beam
through a magnetic sector mass analyser and a collector slit
arranged downstream of the magnetic sector mass analyser, dividing
the ion beam downstream of the collector slit into at least a first
ion beam and a second ion beam, measuring the intensity of at least
a portion of the first ion beam with a first detector and measuring
the intensity of at least a portion of the second ion beam with a
second detector.
[0054] The ion beam has a first direction and a second orthogonal
direction. In the preferred method the ions in the ion beam are
dispersed according to their mass to charge ratio in the first
direction so that the mass to charge ratio of ions in the ion beam
varies along the first direction. Preferably, the ions in the ion
beam are substantially not dispersed according to their mass to
charge ratio in the second direction so that the mass to charge
ratio of ions in the ion beam is substantially constant along the
second direction.
[0055] In the preferred embodiment the first and second detectors
measure the intensities of at least a portion of the first and
second ion beams at substantially the same time. The method
preferably further comprises determining the intensity of at least
a portion of the first ion beam relative to the intensity of at
least a portion of the second ion beam. Preferably, if the
intensity of at least a portion of the first and/or second ion beam
differs from the intensity of at least a portion of the second
and/or first ion beam respectively by a greater than or equal to a
percent x, then a determination may be made that the ion beam
includes a significant proportion of interference ions. The percent
x may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45,
50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 or greater than 50.
Alternatively, or in addition if within a time t the number of ions
detected by the first detector differs from the number of ions
detected by the second detector by greater than or equal to y
standard deviations of the total number of ions detected by the
first and second detectors during the time t, then a determination
may be made that the ion beam includes a significant proportion of
interference ions. Preferably, the number of standard deviations y
is 0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.25, 2.5, 2.75,
3.0, 3.25, 3.5, 3.75, 4.0 or greater than 4.0.
[0056] In a preferred embodiment the method further comprises
summing signals from the first and second detectors to produce a
combined signal and multiplying the combined signal by a weighting
factor. Preferably, the weighting factor does not substantially
attenuate the combined signal when the signal from the first
detector substantially equals the signal from the second detector.
Additionally, or alternatively, the weighting factor may
substantially attenuate the combined signal when the signal from
the first detector substantially differs from the signal from the
second detector. In one embodiment the weighting factor is of the
form exp (-ky.sup.n), where k and n are constants and within a time
t the number of ions detected by the first detector differs from
the number of ions detected by the second detector by y standard
deviations of the total number of ions detected by the first and
second detectors during the time t. In this embodiment the
difference between the number of ions detected by the first and
second detectors is taken as a positive value. Preferably, the
constant k is 0.5-2.0, 0.6-1.8, 0.7-1.6, 0.8-1.4, 0.9-1.2, 0.95-1.1
or 1. Preferably, the constant n is 1.0-3.0, 1.2-2.8, 1.4-2.6,
1.6-2.4, 1.8-2.2, 1.9-2.1 or 2.
[0057] In the preferred method, if a determination is made that the
ion beam includes a significant proportion of interference ions
then the signals from the first and/or second detectors may be
discarded or are otherwise deemed to be relatively inaccurate.
Alternatively, if a determination is made that the ion beam does
not include a significant proportion of interference ions then
signals from the first and second detectors may be summed or
otherwise deemed to be relatively accurate.
[0058] Various embodiments of the present invention together with
other arrangements given for illustrative purposes only, will now
be described, by way of example only, and with reference to the
accompanying drawings in which:
[0059] FIG. 1 shows a conventional single focusing magnetic sector
mass spectrometer;
[0060] FIG. 2 shows a conventional double focusing magnetic sector
mass spectrometer;
[0061] FIG. 3 shows a conventional measurement of
2,3,7,8-tetrachlorinated dibenzo-p-dioxin obtained by High
Resolution Selective Ion Recording;
[0062] FIG. 4 shows an ion detector according to a preferred
embodiment of the present invention;
[0063] FIG. 5 shows an embodiment wherein a lens is used to focus
an ion beam which has passed through a collector slit onto the
entrance aperture of an ion detector according to the preferred
embodiment;
[0064] FIG. 6 shows a particularly preferred embodiment wherein
ions are detected using conversion dynodes in combination with
microchannel plate detectors;
[0065] FIG. 7 shows another embodiment wherein two detectors are
provided on each side of a reflecting electrode;
[0066] FIG. 8 shows an embodiment wherein in a mode of operation
ions may be directed onto a preferred ion detector and wherein in
another mode of operation ions may be deflected onto a second
detector system;
[0067] FIG. 9 illustrates a typical peak profile which may be
observed using a conventional ion detector;
[0068] FIG. 10 illustrates the peak profiles which may be observed
using an ion detector according to the preferred embodiment;
[0069] FIG. 11 illustrates the effect of a small shift in the
position of an ion beam comprising 20 ions incident upon the
collector slit of an ion detector according to the preferred
embodiment having a high resolution of 10,000; and
[0070] FIG. 12 illustrates the effect of a small shift in the
position of an ion beam comprising 100 ions incident upon the
collector slit of an ion detector according to the preferred
embodiment having a low resolution of 2000.
[0071] A conventional single focusing magnetic sector mass
spectrometer is shown in FIG. 1. The mass spectrometer comprises an
ion source 1, a magnetic sector mass analyser 2 and a collector
slit 3 arranged immediately upstream of an ion detector (not
shown). The ion source 1 has a slit 4 which defines the width of an
ion beam emerging from the ion source 1. The magnetic sector mass
analyser 2 shown in FIG. 1 has convergent directional focusing
characteristics. An ion collector slit 3 is positioned at the image
point of the ion source slit 4 so that a single focusing magnetic
sector mass spectrometer is provided. Although the directional
focusing characteristics of the single focusing magnetic sector
mass spectrometer can be designed to a very high order, its imaging
properties will be limited by any spread in the energies of the
ions emitted from the ion source 1.
[0072] FIG. 2 shows a conventional double focussing mass
spectrometer. The mass spectrometer comprises an ion source 1
having a source slit 4. Ions from the ion source 1 pass through a
first electric sector 5 and are brought to a first intermediate
image 6. The ions then pass through the magnetic sector mass
analyser 2 and are brought to a second intermediate image 7 before
passing through a second electric sector 8 prior to the ions being
focused onto a collector slit 3. The electric sectors 5,8 serve to
reduce the dispersion of the ions with different energies which
would otherwise cause the image width to be broadened and which
would hence limit the resolution of the mass spectrometer.
[0073] FIG. 3 shows a conventional measurement of the signal
intensity as a function of retention time in a gas chromatograph
for a solution containing 5 fg of 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin analysed using a conventional double focussing
magnetic sector mass spectrometer having a resolving power of
10,000 (10% valley definition) and set to monitor ions having a
molecular weight of 321.8936. From this measurement it can be seen
that the detection level for 2,3,7,8-tetrachlorinated
dibenzo-p-dioxin is limited by the noise created by other ions
passing through the collector slit, some of which will be compounds
which have the same nominal mass as the analyte. In this example it
can be seen that the detection level is limited to approximately 1
fg (3 atto-mole).
[0074] A preferred embodiment of the present invention will now be
described with reference to FIGS. 4 and 5. A mass spectrometer
according to the preferred embodiment comprises a split ion
detector 11 having two or more separate detectors 14a,14b which are
provided in conjunction with a single collector slit 3 (see FIG.
5). Ions which are transmitted through the single collector slit 3
pass into the split ion detector 11 and are divided in the
direction of mass dispersion by a reflecting electrode 13 to one
side or the other of the reflecting electrode 13 depending upon the
position of the ions and the direction in which the ions are
heading. The ions reflected by the reflecting electrode 13 are
directed onto one of two or more detectors 14a,14b.
[0075] An ion beam which is uniformly distributed across the
collector slit 3 and/or which is distributed symmetrically about
the centre of the collector slit 3 will be divided substantially
equally so that substantially half of the ions in the ion beam will
be reflected by the reflecting electrode 13 so that they are
incident upon one of the detectors 14a;14b whilst the other half of
the ions in the ion beam will be reflected by the reflecting
electrode 13 so that they are incident upon the other detector
14a;14b. Conversely, an ion beam which is not uniformly distributed
across the collector slit 3 and/or which is not distributed
symmetrically about the centre of the collector slit 3 will be
divided by the electrode 13 unequally so that the signal from the
two detectors 14a,14b will be substantially different.
[0076] According to the preferred embodiment the collector slit 3
is precisely positioned so that only analyte ions of interest will
be distributed uniformly across the collector slit 3 and/or will be
distributed symmetrically about the centre of the collector slit 3.
Accordingly, only analyte ions of interest will be distributed
uniformly and/or symmetrically across the reflecting electrode 13
and hence substantially 50% of the analyte ions of interest will be
incident upon one of the detectors 14a;14b whilst substantially 50%
of the analyte ions will be incident upon the other detector
14a;14b. However, interference ions will pass through the magnetic
sector mass analyser on slightly different trajectories and hence
will not be distributed uniformly or symmetrically across the
collector slit 3. Accordingly, the interference ions will not
therefore be distributed uniformly or symmetrically across the
reflecting electrode 13, and hence the interference ions will not
be distributed equally between the two detectors 14a,14b. It
therefore follows that the ion signals from the two detectors
14a,14b will be substantially different. Therefore, by measuring
the relative intensity of the signals from the two detector 14a,14b
it is possible to determine whether or not the total ion beam is
uniformly distributed across the collector slit 3 and/or whether or
not the ion beam is distributed symmetrically about the centre of
the collector slit 3. This in turn enables a determination to be
made as to whether or not the detected ion beam includes a
significant proportion of interference ions. If the signals from
the two ion detectors 14a,14b are substantially identical then the
signals can be summed and recorded, otherwise the signals can be
ignored or discarded. Alternatively, the signals from the two ion
detectors may be summed and multiplied by a weighting factor
preferably in the form exp(-ky.sup.n) where k is preferably 1, n is
preferably 2 and y is the standard deviations of the total number
of ions detected by the first and second detectors during a time t.
The weighting factor preferably has the effect of retaining the
significance of the summed signals when the signals are
substantially similar and attenuating or otherwise substantially
suppressing the significance of the summed signals when the signals
differ in intensity significantly.
[0077] The reflecting electrode 13 preferably comprises a finely
edged blade or wedge shaped electrode. The reflecting electrode 13
is preferably arranged substantially perpendicular to the plane of
the collector slit 3 and substantially parallel to the direction of
the magnetic fields such that it divides the ion beam in the
direction of mass dispersion. The ion beam is preferably divided
into two separate ion beams which are then directed onto two or
more detectors 14a,14b. A high voltage relative to the ion source
is preferably applied to the reflecting electrode 13 so that ions
are repelled away from the reflecting electrode 13 and peel off to
one side or the other depending upon which side of the dead centre
of the blade electrode 13 the ion is positioned and heading.
Adjustment of either the ion beam and/or the reflecting electrode
13 is preferably possible such that the ion beam may be aligned
with the centre of the collector slit 3 and such that ions in the
centre of the ion beam are precisely directed towards the beam
dividing edge of the reflecting electrode 13. All of the ions which
pass into the split ion detector 11 are preferably deflected to one
side or the other of the reflecting electrode 13 such that the ions
are detected by two or more detectors 14a,14b.
[0078] Ions preferably enter the split ion detector 11 through a
screening tube 12 (as shown in FIG. 4) and emerge from the
screening tube 12 to preferably immediately confront the reflecting
electrode 13. The screening tube 12 preferably acts to at least
partially, preferably substantially, shield the ions passing
through the split ion detector 11 from any electric fields
resulting from voltages applied to the detectors 14a,14b. The
retarding electric fields generated by the reflecting electrode 13
cause the ions to peel off and be reflected back to the two or more
detectors 14a,14b arranged either side of the screening tube 12.
The two detectors 14a,14b preferably comprise microchannel plates
having anodes positioned behind the microchannel plates. Each ion
that arrives at one of the microchannel plates results in the
generation of a pulse of electrons which is released such that the
electrons are received on the anode behind the microchannel plate.
Each pulse of electrons which are incident on the anode may be
counted or integrated and then measured using an Analogue to
Digital Converter. The ion detectors 14a,14b may include discrete
dynode electron multipliers or continuous dynode channeltrons as
described in more detail below.
[0079] FIG. 5 shows in more detail the portion of the mass
spectrometer intermediate the collector slit 3 and the split ion
detector 11 according to an embodiment of the present invention. A
lens 9 is preferably provided downstream of the collector slit 3
and upstream of the split ion detector 11. The lens 9 preferably
refocuses the image of the collector slit 3 onto the entrance to
the split ion detector 11. Preferably, the refocused image of the
collector slit 3 is magnified such as to increase the spatial
distribution of the ions passing through the collector slit 3 and
arriving at the split ion detector 11.
[0080] FIG. 6 illustrates a particularly preferred embodiment of
the present invention wherein ions reflected by the reflecting
electrode 13 are accelerated towards and onto two conversion
dynodes 15a,15b. Ions striking the conversion dynodes 15a,15b cause
the conversion dynodes 15a, 15b to generate secondary electrons.
The resulting secondary electrons are then detected using two
detectors 14a,14b which preferably comprise microchannel plate ion
detectors. An advantage of using conversion dynodes 15a,15b to
initially detect the ions rather than microchannel plates is that
the efficiency of ion detection can be increased to near 100%. A
microchannel plate typically has an effective ion receiving area of
60-70% upon which an ion impinging will result in the production of
secondary electrons. Therefore, the ion detection efficiency of a
microchannel plate is effectively limited to approximately 60-70%.
In contrast, conversion dynodes 15a,15b have an ion detection
efficiency of approximately 100% and typically will yield between
two and six electrons per ion incident upon the respective
conversion dynode 15a,15b. Accordingly, the probability that the
microchannel plates 14a,14b arranged as shown in FIG. 6 will detect
at least one of the secondary electrons generated and released by
the conversion dynodes 15a,15b in response to an ion impacting upon
the conversion dynode 15a,15b is virtually 100%.
[0081] According to another less preferred embodiment ions may be
accelerated from the conversion dynodes 15a,15b and be received on
one or more scintillators and/or one or more phosphors (not shown).
The resulting photons may then preferably be detected using one or
more photo-multiplier tubes ("PMT") and/or one or more
photosensitive solid state detectors (not shown).
[0082] FIG. 7 illustrates a further embodiment wherein two
detectors 14a,14c;14b,14d are positioned on each side of the
reflecting electrode 13 so that a total of four ion detectors are
provided. In this embodiment a portion of an ion beam which is
defected to one side of the central reflecting electrode 13 will be
received upon two ion detectors 14a,14c. Similarly, the portion of
the ion beam deflected to the other side of the central reflecting
electrode 13 will be received upon two other detectors 14b,14d.
This embodiment allows any asymmetry of the ion beam with respect
to the reflecting electrode 13 (and hence collector slit 3) to be
more accurately determined. It is contemplated that according to
further unillustrated embodiments six, eight, ten, twelve or any
number of further ion detectors may be provided.
[0083] FIG. 8 illustrates a further embodiment wherein the
preferred split ion detector 11 is provided downstream of a second
detector system 16,17,18. The second detector system 16,17,18 is
preferably provided off-axis with respect to the ion beam so that
neutral particles in the ion beam preferably do not interfere with
the second detector system 16,17,18.
[0084] In this embodiment the upstream ion detector preferably
comprises a conversion dynode 16, one or more focusing ring
electrodes 17, a scintillator (or phosphor) and a photo-multiplier
18. When the voltages applied to the second detector system are
switched OFF the ions travel directly past the second detector
system onto the preferred split ion detector 11 without
interruption. When the voltages applied to the second detector
system are switched ON then ions are preferably deflected onto the
conversion dynode 16. Ions strike the conversion dynode 16 and
cause secondary electrons to be released which are then preferably
accelerated and focused onto the scintillator or phosphor 18 by the
one or more ring lenses 17. Alternatively, the ions may be
deflected directly onto a microchannel plate detector (not
shown).
[0085] In an alternative embodiment the preferred split ion
detector 11 and the second detection system 16,17,18 may be
arranged such that ions may be directed to one or other of the two
detectors by an electrostatic and/or magnetic field.
[0086] In a further embodiment, in one mode of operation
substantially all of the ions may be directed onto one of the
detectors 14a,14b,14c,14d of the preferred split ion detector 11 by
an electric and/or magnetic field.
[0087] When an ion beam comprising ions having a specific mass to
charge ratio is scanned across the collector slit 3 the resulting
signal profile is commonly referred to as the peak profile. As the
ion beam is scanned across the collector slit 3 ions will begin to
be onwardly transmitted to the ion detector when the leading edge
of the ion beam reaches a first edge of the collector slit 3. Ions
will then continue to be transmitted through the collector slit 3
and to the ion detector until the trailing edge of the ion beam
arrives at the second opposite edge of the collector slit 3.
Accordingly, the width of the peak profile will be the width
w.sub.b of the ion beam summed with the width w.sub.e of the
collector slit. If the width w.sub.b of the ion beam is
substantially equal to the width w.sub.c of the collector slit 3
then the peak profile will have a maximum corresponding to when the
ion beam is symmetrically distributed about the centre of the
collector slit 3. The peak profile will vary depending upon the
relative width w.sub.b of the ion beam and the width w.sub.c of the
collector slit 3. The peak profile will also vary depending upon
the ion intensity profile of the ion beam.
[0088] High Resolution Selected Ion Recording measurements, as
described above for the detection of traces of
2,3,7,8-tetrachlorinated dibenzo-p-dioxin, are commonly carried out
at a mass resolution of 10,000 (10% valley definition). A mass peak
that has a width of 100 ppm of the mass when measured at 5% of the
maximum intensity will have a mass resolution of 10,000 (10% valley
definition). A mass peak that is 100 ppm wide will usually have
maximum transmission when the collector slit width w.sub.e is just
equal to that of the ion beam width w.sub.b i.e. when the collector
slit 3 and the ion beam each have a width of 50 ppm. Under these
conditions the source slit width w.sub.s is as large as it can be
for the collector slit 3 to just transmit the total beam arriving
at the collector slit 3 and for the peak width (w.sub.b+w.sub.c) of
100 ppm.
[0089] FIG. 9 shows an example of the peak profile P obtained when
an ion beam B having a beam width w.sub.b of 50 ppm is scanned
across a collector slit 3 having a width w.sub.c of 50 ppm. The
resulting observed peak profile P will have a width of 100 ppm and
will have a maximum corresponding to when the ion beam is centred
on the collector slit 3. The ion beam profile B is shown at a
position centred on the collector slit 3. The ion beam profile B
may vary according to a number of parameters in the design of the
mass spectrometer although a typical beam profile may follow a
cosine distribution. In the example illustrated in FIG. 9 the ion
beam profile B has a cosine distribution and the resulting observed
peak profile P detected by a conventional single ion detector has a
cosine squared distribution.
[0090] In High Resolution Selected Ion Recording experiments the
ion beam is switched to a central position where substantially 100%
of the ion beam is transmitted through a collector slit of the mass
spectrometer. Since the ion beam is not scanned across the
collector slit then this approach does not allow any knowledge of
the peak profile to be gained. The peak profile can only be assumed
to be that as shown, for example, by P in FIG. 9. If the peak
profile is not as expected, for example, due to the peak not having
precisely the right mass to charge ratio or because the peak
includes the measurement of randomly scattered ions having very
slightly differing mass to charge ratios then this will not be
known and the interference ions will be included in the measurement
of the analyte ions. If, however, the ion beam that is transmitted
through the collector slit is split into two or more ion beams
which are detected on two or more detectors as according to the
preferred embodiment then the situation is quite different as will
be shown in more detail below.
[0091] FIG. 10 shows an example of the peak profiles
P.sub.1,P.sub.2,P.sub.sum which will be observed where an ion beam
having a profile B and a width w.sub.b of 50 ppm is incident upon a
collector slit 3 having a width w.sub.c of 50 ppm and is detected
using a split ion detector 11 according to the preferred
embodiment. The resulting peak profiles P.sub.1,P.sub.2 recorded on
the two detectors of the preferred split ion detector 11 are each
75 ppm wide and are displaced by 25 ppm with respect to each other.
If the two peak profiles P.sub.1,P.sub.2 are summed then the
resulting peak profile P.sub.sum will be 100 ppm wide and will have
substantially the same profile as that recorded on a conventional
single ion detector as shown in FIG. 9.
[0092] In a High Resolution Selected Ion Recording experiment
wherein the ion beam is switched to the central position, the ion
signal recorded on each of the two detectors of the preferred split
ion detector 11 will be substantially the same provided that the
ion beam is symmetrically disposed about the centre of the
collector slit 3. If, however, the peak profile is not as expected
because, for example, the ions include interference scattered ions
having slightly different mass to charge ratios or because the ions
include randomly scattered ions having similar mass to charge
ratios, then the ion signals detected by the two detectors of the
preferred split ion detector 11 will not be equal. Therefore, the
split ion detector 11 of the preferred embodiment enables a
determination to be made as to whether or not (and indeed to what
extent) interference ions are being detected together with the
desired analyte ions and hence whether or not the ion signal is
reliable. Equally, if the ion beam is substantially free from the
presence of interference ions then the ion signal from the two
detectors will be substantially equal and it can be concluded to a
high degree of confidence that the intended analyte ions are being
detected without undesired interference ions affecting the
measurement of the intensity of the analyte ions.
[0093] It will be seen from FIG. 10 that when the ion beam is
switched to the central position each detector of the preferred
split ion detector 11 is not detecting the maximum number of ions
that it would detect if the ion beam were shifted by 12.5 ppm. The
ion beam is not therefore positioned at the peak top for either of
the two detectors of the preferred split ion detector 11 even
though it is positioned at the peak top of the peak profile
P.sub.sum for the sum of the peak profiles P.sub.1,P.sub.2 of the
individual detectors. This means that a very small shift in the
position of the ion beam will cause the signal on one of the
detectors to increase whilst the signal on the other detector will
simultaneously decrease. Hence, the preferred split ion detector 11
is very sensitive to small shifts in the position of the ion beam
and very sensitive to the presence of interference ions.
[0094] The effect of a small shift in the position of the ion beam
will be further illustrated with reference to FIG. 11. With the
table shown in FIG. 11 it is assumed that the resolution of the
preferred ion detector 11 is 10,000 (10% valley definition) and
that only 20 ions are transmitted through the collector slit 3 and
are subsequently detected by the preferred split ion detector 11.
In this illustration the ion beam and the collector slit 3 both
have a width of 50 ppm resulting in an observed peak profile width
of 100 ppm.
[0095] Column 1 of FIG. 11 tabulates a series of shifts in the ion
beam away from the centre of the collector slit in units of ppm.
Column 2 tabulates the corresponding number of ions that would be
detected on the first detector of the preferred split ion detector
11 for the corresponding shift in the ion beam detailed in column
1. Column 3 similarly tabulates the number of ions that would be
detected on the second detector for the same corresponding shift in
the ion beam.
[0096] Column 4 tabulates the total number of ions detected by the
first and second detectors, i.e. the sum of columns 2 and 3. It can
be seen that as the position of the ion beam is increasingly
shifted away from the centre then the total number of ions detected
is reduced. This is because the ion beam and the collector slit 3
are the same width and as the ion beam is moved off centre not all
of the ions in the ion beam will be incident upon the collector
slit 3 and hence not all of the ions will be onwardly
transmitted.
[0097] Column 5 tabulates the average number of ions that would
have been expected to have been detected on each of the first and
second detectors had the ion beam been positioned on the centre
given the total ion count reported in column 4. In other words,
column 5 simply reports half the total number of ions reported in
column 4 for each value of ion beam shift. Column 6 tabulates one
standard deviation for the expected ion count for each of the first
and second detectors which is reported in column 5.
[0098] Column 7 tabulates the difference between the actual ion
count for the first detector reported in column 2, and the ion
count that would have been expected as reported in column 5,
expressed in terms of the number of standard deviations of the
expected ion count tabulated in column 6. Column 8 similarly
tabulates the difference, in terms of standard deviations, between
the actual ion count for the second detector reported in column 3,
and the expected count reported in column 5, again expressed in
terms of the number of standard deviations of the expected ion
count tabulated in column 6.
[0099] Column 9 tabulates the percentage probability Pi for the
difference in ion count from the expected average being equal to or
less than the actual difference in ion count reported for the first
detector in column 7 assuming a natural or Gaussian distribution.
Likewise, column 10 tabulates the same percentage probability P2
for the difference in ion count reported for the second detector in
column 8. Hence columns 9 and 10 report the percentage probability
of observing measurements within the relative standard deviations
reported in columns 7 and 8 respectively, were an ion beam having
the number of electrons reported in column 4 centred on the
collector slit. Finally, column 11 tabulates the combined
percentage probability P of both observing a measurement outside of
the relative standard deviation reported in column 7 and a
measurement outside of the relative standard deviation reported in
column 8. In other words, column 11 reports the percentage
probability of observing the two ion counts recorded on the first
and second detectors for a peak having a total ion count equal to
the sum of the two separate ion counts and positioned
centrally.
[0100] It will be seen from FIG. 11 that the ion counts on the two
detectors corresponding to an ion beam comprising only 20 ions and
wherein the beam of ions is shifted by just 5 ppm is such that the
probability that the observed ion counts could be observed if the
ion beam were positioned centrally is only approximately 13%.
Furthermore, the ion counts on the two detectors from an ion beam
comprising only 20 ions wherein the ion beam is shifted by 10 ppm
is such that the probability that the observed ion counts would be
observed if the ion beam were positioned centrally is only 1%.
Similarly, if the ion beam is shifted by 15 ppm then the
probability that the observed ion counts would be observed if the
ion beam were positioned centrally is only 0.1%.
[0101] In this example, it is apparent that the benefit of using a
split ion detector according to the preferred embodiment is such
that for the measurement of an ion beam comprising just 20 ions it
could be ascertained with a 99% confidence that the observed mass
peak is not an interfering peak due to the ion beam being displaced
by only 10 ppm. Alternatively, it could be ascertained with a 99.9%
confidence that the observed mass peak is not an interfering mass
peak due to the ion beam being displaced by 15 ppm.
[0102] In contrast, using a conventional ion detector it would be
necessary to operate with a mass peak width at 5% height of
approximately 20 ppm to achieve the same specificity. This would
correspond to an extremely high resolution of approximately 50,000
(10% valley definition) in contrast to 10,000 according to the
preferred embodiment. Therefore, in this example, it is apparent
that the split ion detector according to the preferred embodiment
provides approximately a five-fold increase in specificity compared
to a comparable conventional ion detector.
[0103] Alternatively, the preferred split ion detector may be
considered as providing the same specificity but being between 5
and 25 times more sensitive as this is the likely loss in
sensitivity resulting from increasing the resolution of the mass
spectrometer five fold from 10,000 to 50,000 (10% valley
definition).
[0104] FIG. 12 shows another example of the effect of small shifts
in the position of an ion beam incident upon a preferred ion
detector. The resolution of the preferred ion detector in this
example has been reduced to 2000 (10% valley definition). As a
consequence it has been assumed that the transmission has been
increased by a factor of five so that the total number of ions
detected by the preferred split ion detector has increased to 100.
In this illustration the ion beam width and the collector slit 3
width are both 250 ppm resulting in a peak width of 500 ppm. It
will be seen from FIG. 12 that the ion counts on the two detectors
due to the ion beam being shifted by 20 ppm from the centre are
such that the probability that the observed ion counts could be
observed if the ion beam were positioned centrally on the collector
slit is only 1%. This example illustrates the benefit of using a
split ion detector according to the preferred embodiment since for
the measurement of a peak corresponding to 100 ions at a resolution
of 2000 (10% valley definition) it could be ascertained with a 99%
confidence that the peak is not an interfering peak displaced by
only 20 ppm. In contrast, using a conventional ion detector it
would be necessary to operate with a peak width at 5% height of 40
ppm to achieve the same specificity. This corresponds to a high
resolution of 25,000 (10% valley definition) compared to a
resolution of just 2000 according to the preferred embodiment. It
also follows that the preferred split ion detector will have both
an increased sensitivity and an increased specificity compared to
that achievable with a conventional ion detector operating at a
resolution of 10,000 (10% valley definition).
[0105] It has been shown that the split ion detector according to
the preferred embodiment can either improve the specificity of mass
analysis without loss in sensitivity, or can provide an improved
sensitivity without loss in specificity, or indeed can provide both
improved sensitivity and specificity. Furthermore, randomly
scattered ions that constitute background noise can at least be
partially if not substantially eliminated.
[0106] Although the present invention has been described with
reference to preferred embodiments, it will be understood by those
skilled in the art that various changes in form and detail may be
made without departing from the scope of the invention as set forth
in the accompanying claims.
* * * * *